This invention relates to temperature regulating PTCR heaters and more particularly to such a heater wherein the electrodes and means for contacting the electrodes are on one side of a PTCR ceramic plate and an electrically insulative film is bonded directly to the other side of the plate. A ceramic PTCR heater plate having a pair of interdigitated electrodes confined to one major surface of the plate is described in the patent to Fabricius U.S. Pat. No. 3,885,129 issued May 20, 1975 that is assigned to the same assignee as is the present invention. This plate is illustrated in the patent in FIGS. 5 through 9 and shows an object to be heated being spaced from the one electroded surface by an insulative layer. When a source of electrical energy is connected between the two electrodes current flows predominently at that one major surface between the electrodes. Thus the object to be heated is only separated from the central source of heat by the interposed electrically insulative film. Contact pads for the electrodes are thus placed on the opposite major surface of the plate to avoid further separation of the heated object from the heating surface of the PTCR ceramic.
The electrodes are typically formed by screen printing a metal paste on the one major surface of the plate. The termination pads on the opposite surface may likewise be screen printed. Connection between them is made by applying a strip of the metal containing paste to the plate edges. The plate is then heated to cure the electrodes, edge coatings and termination pads.
However, it has been found that the edge coatings constitute a major cause of failure since the edge coatings tend to become very thin at the corners and causes the electrical current density to be excessively high at these edges. Consequently this electrical connection tends to overheat and open the circuit. The problem may be ameliorated by grinding the ceramic plate edges to round them prior to applying the conductive edge coatings, but at additional cost.
It is an object of this invention to provide a PTCR heater requiring no edge connections and with all metallizations confined to one major PTCR plate surface.
It is a further object of this invention to provide a reliable low cost self regulating heater.